The Pacific Transition pattern is prominent between May-August. The mode consists of a wave-like pattern of height anomalies, which extends from the Gulf of Alaska eastward to the Labrador Sea, and is aligned along the 40oN latitude circle. The prominent centers of action have a similar sign, and are located over the ... intermountain region of the United States and over the Labrador Sea. Relatively weak anomaly centers with signs opposite to the above are located over the Gulf of Alaska and over the eastern United States.

Two of the most pronounced negative phases of the PT pattern in the historical record occurred during July 1992 and July 1993. During each of these periods, well below-normal 500-mb heights were observed over the northwestern United States, in association with a substantially reduced strength of the climatological mean ridge, which is located over this region in Summer. Below-normal heights were also observed over the Canadian Maritime Provinces and over the central North Pacific during these months, while above-normal heights were observed over the Gulf of Alaska. During July 1993, these extremely anomalous conditions were associated with a continuation of record flooding throughout the Midwest United States (Bell and Janowiak 1995).